Three new selenophene-based conjugated copolymers having different ratios of the monomeric units were designed, synthesized and thoroughly characterized. The introduction of an electron-poor and surfaced building moiety like selenathiazole was highly efficient in tuning the bandgap and polymer properties. The chalcogenophene-based medium-bandgap polymers demonstrated low-lying HOMO energy levels (∼5.87 eV), which is benign for use in multi-junction polymer solar cell applications. The representative polymers with heavy atoms revealed the change in electronegativity and atomic size that highly affected the molecular property, its topological features, and photovoltaic properties in polymer solar cells. The selenium-substituted (0.5:0.5) polymer donors showed power conversion efficiencies above 3% when combined with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₀BM) acceptors in a quintessential bulk-heterojunction solar cell.